feat(mcp,acp): Repository.WithTx + acp.Repository.InTx

Cross-module transactional support for spec §6.1: ACP CreateSession must
atomically insert acp_sessions row + mcp_tokens row (FK NOT NULL).

- mcp.Repository.WithTx(tx) -> Repository (tx-bound, no pool)
- acp.Repository.InTx(ctx, fn) -> error (pgx.BeginFunc wrapper)
- acp.Repository.WithTx(tx) -> Repository (tx-bound)
- Update fakeRepos in test files to satisfy new interface methods

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/acp/relay_test.go

@@ -18,6 +18,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
@@ -112,6 +113,14 @@ func (f *fakeEventRepo) PurgeEventsBefore(context.Context, time.Time) (int64, er
 	panic("n/a")
 }
 
+func (f *fakeEventRepo) InTx(context.Context, func(context.Context, pgx.Tx) error) error {
+	panic("n/a")
+}
+
+func (f *fakeEventRepo) WithTx(pgx.Tx) acp.Repository {
+	return f
+}
+
 // relayTestRig wires up a Relay with two io.Pipe pairs that simulate the agent
 // subprocess. Returns:
 //   - r:           the relay under test